The present invention relates to a rotary anode for use in a rotary anode type X-ray tube which is required to have a high X-ray output and a method for manufacturing the rotary anode.
With the increasing performance of medical equipment such as an X-ray CT, a more powerful X-ray output is required. Thus, a large heat capacity as well as light weight is required for a rotary anode for use in an X-ray tube. In order to attain this purpose, it has been proposed to employ, in place of a rotary anode made of metal only, a structure comprising a graphite substrate and an X-ray generating metal plate bonded to the substrate or a graphite substrate plated with an X-ray generating metal.
The former structure in which an X-ray generating metal plate is bonded to a graphite substrate is made by bonding an X-ray generating metal plate produced to the graphite substrate plate by brazing or the like (first prior art method). In the latter structure comprising a graphite substrate plated with an X-ray generating layer, the X-ray generating layer is formed by chemical vapor deposition on the graphite substrate, for example as disclosed in Japanese Patent publication 47-8263 (second prior art method). The publication also teaches the formation of an intermediate layer of rhenium as a diffusion shield to prevent mutual diffusion between tungsten, the best material as an X-ray generating layer, and the graphite substrate. Further, it is known to add rhenium to tungsten to prevent the X-ray generating intensity from dropping. Further, as disclosed in Japanese Unexamined Patent Publication 59-8252, it is known to subject the assembly to heat treatment in a non-oxidizing atmosphere after forming the intermediate layer and the X-ray generating layer by the second prior art method to form alloy layers between the adjacent layers and thus to improve adhesion between layers (third prior art method).
But the first prior art method has a problem in that the bonding of the X-ray generating metal plate is unstable. Further the metal plate has to be made unnecessarily thick for structural reasons, so that the weight of the entire rotary anode cannot be markedly reduced.
The second prior art method is advantageous over the first prior art method in that a good adhesion is achieved between the graphite substrate and the X-ray generating layer, so that the thickness of the X-ray generating layer having a large specific gravity can be reduced to a minimum. But since the intermediate layer of rhenium has a columnar grain structure, mutual diffusion tends to occur between the tungsten and the graphite substrate. Such diffusion will cause the formation of brittle tungsten carbide under the X-ray generating layer. This may in turn cause reduced X-ray output or peeling or cracking of the X-ray generating layer. Thus, this method is not suitable for prolonged use.
With the third prior art method, a tungsten carbide layer is formed under the X-ray generating layer during the heat treatment step. Thus, this method cannot offer an effective solution to the problems encountered by the second method.